Other meanings of Local Group
Astronomy
The Local Group is the galaxy group that contains the Milky Way and the Andromeda Galaxy (M31), along with more than 80 other known galaxies, bound together by gravity and spanning roughly 10 million light-years.1 It is a typical example of a small galaxy group, and it forms part of the larger Virgo Supercluster.2 The group's two largest members dominate its dynamics, and their mutual gravitational attraction is drawing them together, with a merger expected in about 4.5 billion years.3
The Local Group is defined by its gravitational binding: galaxies that are mutually attracted and moving together against the cosmic expansion. The group is centered roughly between the Milky Way and Andromeda, with most members falling into two subgroups around these giants.1 The Triangulum Galaxy (M33) is the third-largest member, and it is likely a satellite of Andromeda. Beyond the two dominant spirals, the group contains a variety of dwarf galaxies, including irregulars like the Magellanic Clouds, ellipticals like M32, and numerous ultra-faint dwarf spheroidals discovered in recent sky surveys.4 Membership is determined by a combination of radial velocity, distance, and tidal stability, and the exact census continues to grow as deeper surveys reveal fainter systems.
The Local Group is not static; its galaxies are in orbit around a common barycenter, and the dominant interaction is the mutual attraction between the Milky Way and Andromeda. Measurements of Andromeda's transverse motion using the Hubble Space Telescope show that it is moving toward the Milky Way at about 110 km/s, implying a head-on collision in roughly 4.5 billion years.3 This merger will likely produce a giant elliptical galaxy, sometimes nicknamed 'Milkomeda.' The group's total mass is estimated at about 2×10^12 solar masses, but the distribution is uncertain, with dark matter dominating.5 The group is also accreting new members, such as the recently discovered Antlia 2 and Crater 2 dwarfs, which are among the most diffuse galaxies known.4
Besides the Milky Way and Andromeda, the Local Group contains several notable galaxies. The Large and Small Magellanic Clouds are the largest satellite galaxies of the Milky Way and are visible to the naked eye from the Southern Hemisphere. M32 and M110 are prominent satellites of Andromeda, and M33 is a face-on spiral that serves as a key laboratory for star formation. The group also includes the intriguing dwarf galaxy IC 10, a starburst galaxy that is the only known starburst in the Local Group.6 In 2018, the Antlia 2 galaxy was discovered, which is as large as the Large Magellanic Cloud but has a surface brightness so low that it had escaped detection for decades.4 These diverse members provide a sample of galaxy types and evolutionary stages.
The Local Group's boundaries are not sharp; the group is defined by a velocity cutoff, and some galaxies are in a 'halo' region where membership is ambiguous. For example, the galaxy IC 1613 is sometimes considered a member, but its distance and velocity place it near the edge.1 The group also contains a number of ultra-faint dwarf galaxies that were only discovered in the last two decades, such as Tucana II and Boötes I, which are among the oldest and most metal-poor objects in the universe.4 The Sagittarius Dwarf Spheroidal Galaxy is currently being tidally disrupted by the Milky Way, providing a real-time example of galactic cannibalism. Additionally, the Local Group is moving relative to the cosmic microwave background at about 600 km/s, a motion that reflects the influence of large-scale structure.2
The Local Group is one of many galaxy groups in the observable universe, and its study provides insights into galaxy formation and evolution.
Help improve the encyclopedia. Reports go straight to the site manager.